Trunking in a matrix

ABSTRACT

A multistage switch includes a matrix of coupled switch devices. A logical link comprising a plurality of physical links couples a destination through the plurality of physical links to a plurality of ports in the multistage switch. Each switch device performs trunk aware forwarding to reduce the forwarding of received frames through the matrix of coupled switch devices to the destination in order to reduce unnecessary traffic in the multistage switch.

RELATED APPLICATION

This application is a continuation of U.S. application Ser. No.10/066,550, filed Jan. 31, 2002 now U.S. Pat. No. 7,313,135. The entireteachings of the above application are incorporated herein by reference.

BACKGROUND OF THE INVENTION

A networking switch device receives frames through a number of ingressports connected to the switch device and forwards the frames through anumber of egress ports connected to the switch device. The switch devicedetermines one or more egress ports through which the received framesare forwarded dependent on the destination address included in theframe.

A logical link can be configured to increase the bandwidth to adestination; that is, the number of frames that can be forwarded throughthe switch device. A logical link includes a plurality of egress portsin the switch through which frames can be forwarded to the destination.Configuring multiple ports to forward frames to a destination is calledlink aggregation or trunking. The egress ports in the logical link areconfigured as members of the logical link or the trunk group.

A multistage switch includes a plurality of coupled switch devices. Onewell-known method for providing a multistage switch is the 3-stage Closnetwork. The multistage switch includes external ports for receivingframes from a source and forwarding frames to a destination. Each switchdevice in the multistage switch (or matrix) includes internal ports forforwarding frames within the matrix.

Typically, a frame received from a source at one external port isforwarded through the switch devices in the multistage switch to anotherexternal port dependent on a destination address associated with theframe. A switch device in the matrix forwards a received frame to one ofits output ports dependent on a destination address included in theframe. The destination address may be for a host such as a computer,another multistage switch or a router.

SUMMARY OF THE INVENTION

To provide redundancy, a logical link can include external ports ondifferent switch devices in the multistage switch. The selected memberof a trunk group for a frame arriving at an external port in themultistage switch may be on the switch device receiving the frame or onanother switch device.

Each switch device forwards a received frame based on local forwardinginformation stored in a forwarding table in the respective switchdevice. When forwarding a frame to a trunk group, the frame is forwardedto each switch device having a member of the trunk group. For example,if members of a trunk group can be reached through internal ports 0 and1 of the switch device, the frame is forwarded through both ports. Thus,a frame may be needlessly forwarded throughout the matrix of switchdevices before being forwarded to the selected member of the trunkgroup, resulting in unnecessary traffic in the matrix. Unnecessarytraffic in the matrix of switch devices can be reduced by performingmatrix trunk aware forwarding in each switch device.

A logical link including a plurality of physical links, i.e. a trunkgroup, couples a destination to a multistage switch. The multistageswitch includes a plurality of external ports. Each physical link iscoupled to one of the plurality of external ports. The multistage switchalso includes a matrix of coupled switch devices. A frame received forthe destination is forwarded through at least one of the switch devicesto one of the physical links in the logical link. The switch devicewhich receives the forwarded frame forwards the frame based on thelogical link to reduce the number of subsequent switch devices throughwhich the frame is forwarded.

The switch device also includes a trunk table selector which selects atrunk table for the logical link to reduce the number of ports of theswitch device through which to forward the frame. The trunk table forthe logical link may be shared by another logical link.

The switch device may also include flow hash logic which indexes a flowhash for the received frame dependent on a destination address and asource address included in the received frame. The switch device mayalso include an echo suppression table. The echo suppression tableincludes an entry for each port. The entry is selected dependent on theport receiving the frame. The selected entry ensures that the frame isnot forwarded to any member of the logical link on which it wasreceived. The echo suppression table includes an entry for each externalport at which a frame is received. The physical links may be coupled toexternal ports on different switch devices.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other objects, features and advantages of theinvention will be apparent from the following more particulardescription of preferred embodiments of the invention, as illustrated inthe accompanying drawings in which like reference characters refer tothe same parts throughout the different views. The drawings are notnecessarily to scale, emphasis instead being placed upon illustratingthe principles of the invention.

FIG. 1 illustrates a multistage switch including a logical linkconnecting a destination to the switch according to the principles ofthe present invention;

FIG. 2 illustrates a logical representation of the switch devices in anyof the rows shown in FIG. 1;

FIG. 3 illustrates unnecessary traffic generated in a multistage switchwithout matrix trunk aware forwarding;

FIGS. 4A-4B illustrate forwarding tables in rows of the multistageswitch shown in FIG. 3;

FIG. 5 illustrates matrix trunk aware forwarding according to theprinciples of the present invention;

FIG. 6A-6B illustrate the forwarding tables providing trunk awareforwarding in rows of the multistage switch devices shown in FIG. 5;

FIG. 7 illustrates the pruning of a frame in the fan-in/fan-out devicein which the frame was received;

FIG. 8A illustrates a frame which may be received at any of the externalports in the multistage switch shown in FIG. 1;

FIG. 8B illustrates the format of an Ethernet data link (L2) headershown in FIG. 8A;

FIG. 8C illustrates the format of an IP network layer (L3) header shownin FIG. 8A;

FIG. 9 illustrates vectors and tables in each of the switch devicesshown in FIG. 2

FIG. 10A illustrates two trunk groups sharing a single trunk table, eachof the trunk groups having a different hash code distribution;

FIG. 10B illustrates the two trunk groups shown in FIG. 10A with eachtrunk group having the same hash code distribution; and

FIG. 11 illustrates an example of local echo suppression tables forreducing unnecessary traffic through the matrix for received unicastframes.

DETAILED DESCRIPTION OF THE INVENTION

A description of preferred embodiments of the invention follows.

FIG. 1 illustrates a logical link or trunk group 134 connecting adestination 112 to a multistage switch 100 according to the principlesof the present invention. The multistage switch 100 includes a matrix ofcoupled switch devices 106-1, . . . 106-2n, 108-1, . . . 108-n, 110-1, .. . 110-n, 114-1, . . . 114-n organized in rows and columns. Each row104-1, . . . 104-n includes five switch devices. As shown, row 104-1includes two fan in/fan out devices 106-1, 106-2, a stage-1 device108-1, a stage-2 device 110-1 and a stage-3 device 114-1. To provideredundancy especially, members of trunk group 134 are distributed amongthe rows in the multistage switch.

Each fan in/fan out device 106-1, . . . 106-2n is coupled to a pluralityof the external switch ports 132-0, . . . 132-12n in the multistageswitch. Each external switch port 132 receives and transmits frames. Areceived frame is forwarded by the respective fan-in/fan-out device onone of six uplink ports 140 to a stage-1 device 108. A frame to betransmitted from the multistage switch is received by the respectivefan-in/fan-out device from a stage-3 114 device on one of six downlinkports 142.

A unicast frame received on any one of the external ports 132 isforwarded by at least one of the matrix of switch devices to anotherexternal port 132 dependent on the source and destination addressincluded in the frame. Echo suppression is performed in each switchdevice to prevent the unicast frame from being transmitted through theexternal port on which it was received. A unicast frame received atexternal port 132-0 coupled to fan-in/fan-out device 106-1 is forwardedto a destination address coupled to external port 132-12n by forwardingthe frame through fan-in/fan-out device 106-1, stage-1 device 108-1, oneof the plurality of a stage-2 devices, a stage-3 device 114-n andfan-in/fan-out device 106-2n in the multistage switch. A unicast framereceived at port 132-1 for destination 112 is first forwarded throughfan-in/fan-out device 106-1. The frame is then forwarded to stage-1device 108-1. Stage-1 switch device 108-1 forwards the frame based on aflow hash dependent on the source and destination address included inthe frame. The IP destination address included in the frame is used togenerate an Egress Port Vector (EPV) to select one of the stage-2devices 110-2n. The selected stage-2 device 110-2n forwards the frame toone of the stage-3 devices 114-1 . . . 114.2n dependent on the taggedEPV. The selected stage-3 device forwards to one of the fan-in/fan-outdevices coupled to the destination 112 dependent on the destinationaddress included in the frame.

In the embodiment shown, the multistage switch 100 includes n rows and12n ports 132-0, . . . 132-12n. Each row has two fan-in/fan-out devicesand each fan-in/fan-out device includes six ports. Ports 132-1, 132-3,132-5, 132-12, 132-(12n-11), 132-(12n-10) are members of trunk group134. A frame received for destination 112 from source 102 a at port132-9 or received from source 102 b at port 132-(12n-1) can be forwardedto destination 112 through any one of the external ports 132 which aremembers of trunk group 134.

An external port in trunk group 134 is selected for forwarding areceived frame dependent on the data flow; that is, the source addressand destination address included in the received frame. By selecting amember of the trunk group 134 dependent on a source address anddestination address, frames for the same data flow are always forwardedthrough the same external port and thus are not mis-ordered through themultistage switch 100.

For example, a frame received by the multistage switch from source 102 aat port 132-9 for destination 112 includes the source address for source102 a and the destination address for destination 112. The multistageswitch 100 determines the data flow from the source address for source102 a and destination address for 112 stored in the frame. One of themembers of trunk group 134 is selected dependent on the data flow.

The forwarding of the frame through the switch devices in the multistageswitch 100 creates traffic in the matrix of switch devices. This trafficis necessary for forwarding a frame received on an external port on oneswitch device to an external port on another switch device. However, ifthe selected member of a trunk group 134 is coupled to fan-in/fan-outdevices in the same row 104-1 as the external port receiving the frame,forwarding the frame through all of the stages of the multistage switchis not necessary. Thus, each switch device needs to perform trunk awareforwarding to reduce the number of frames forwarded through the matrixof switch devices.

A frame received by a fan-in/fan-out device 106 is forwarded to therespective stage-1 device 108 coupled to the fan-in/fan-out device. Uponreceiving the forwarded frame, the respective stage-1 device forwardsthe received frame dependent on the destination address included in theforwarded frame. The stage-1 device generates an Egress Port Vector(EPV) dependent on the included destination address and forwards the EPVin a switch matrix tag included with the frame to a stage-2 device.

Each stage-2 device 110 forwards the received frame to a stage-3 device114 dependent on the EPV included in the switch matrix tag forwarded bythe stage-1 device 108. Each stage-3 device 114 forwards the frame basedon an address lookup dependent on the destination address. Each stage-3device also performs echo suppression dependent on the external portthrough which the frame was received using the number of the multistageswitch source port included in the switch matrix tag forwarded with theframe. By performing echo suppression, the forwarded frame is dropped ifthe frame is received on a member of the logical link coupling thedestination in the multistage switch.

The stage-2 devices 110 do not participate in the forwarding decisionfor the frame through the multistage switch 100 because they merelyforward dependent on the EPV forwarded from the stage-1 devices 108.Hence, the operation of the stage-2 devices 110 can be ignored whendescribing the forwarding of frames through the matrix of switchdevices.

FIG. 2 illustrates a logical representation of the switch devices in row104-1 shown in FIG. 1. To simplify the description of the operation ofthe multistage switch 100, the number of ports in each switch device isreduced.

Fan-in/fan-out switch devices 106-1, 106-2 shown in FIG. 1 arerepresented as respective logical fan-in/fan-out switch devices 200-1,200-2. The six external ports in fan-in/fan-out switch devices 106-1,106-2 are reduced to three external switch ports labeled 0, 1 and 2 inlogical fan-in/fan-out switch devices 200-1, 200-2. The six internaloutput ports in fan in/fan out switch devices 106-1 and 106-2 arereduced to a single uplink port labeled U and the six internal inputports are reduced to a single downlink port labeled D.

The stage-1 device 108-1 and the stage-2 device 110-1 have been combinedto provide ingress device 202-1 because, as discussed already, thestage-2 device 110-1 does not participate in the forwarding decision.The ingress device 202-1 has two uplink ports labeled U0 and U1 andthree internal output ports labeled 0, 1 and 2. The internal outputports are coupled to internal input ports in an egress device 204-1corresponding to the stage-3 device 114-1 of FIG. 1.

The internal output ports on the egress device 204-1 are reduced to twodownlink ports labeled DO and D1. The downlink ports are coupled torespective downlink ports on the logical fan-in/fan-out devices 200-1,200-2. The internal input ports on the egress device 204-1 are reducedto three internal input ports.

FIG. 3 illustrates unnecessary traffic in a multistage switch 100without trunk aware forwarding. Multistage switch 100 has eighteenexternal ports (Ports 0-17). Each respective row 104-1, . . . 104-3 ofswitch devices includes six external ports coupled to logicalfan-in/fan-out devices. Row 104-1 includes logical fan-in/fan-outdevices 200-1, 200-2, an ingress device 202-1 and an egress device204-1. Uplink port U of logical fan-in/fan-out device 200-1 is coupledthrough connection 300-1 to uplink port U0 on ingress device 202-1.Downlink port D of logical fan-in/fan-out device 200-1 is coupledthrough connector 300-4 to downlink port D0 on egress device 204-1. Theinternal output ports (0, 1, 2) on ingress device 202-1 are coupled torespective internal input ports on egress device 204-1 in a first row104-1, on egress device 204-2 in a second row 104-2 and on egress device204-3 in a third row 104-3.

A trunk group T1 in the multistage switch 100 is configured to includeexternal port 0, port 2, port 4, port 12, port 16 and port 17. The trunkgroup T1 defines multiple physical paths to a destination (not shown)through external ports which are members of trunk group T1. Thedestination can be another multistage switch. Frames received by themultistage switch for the destination can be forwarded through any ofthe members of trunk group T1. The member of the trunk group throughwhich to forward a received frame can be selected dependent on acomputed hash of a source and destination address included in the headerof the received frame so that frames for the same data flow areforwarded through the same external port.

Each member of trunk group T1 is assigned a matrix assigned hash code orset of hash codes. Each switch device in the multistage switch 300creates a computed hash code for a received frame dependent on thesource and destination address included in a frame. A switch devicereceiving the frame forwards the frame towards the trunk member whosematrix assigned hash code matches this computed hash code. Trunk groupT1 has six members. Each member of trunk group T1 has an associatedmatrix assigned hash code.

As shown in FIG. 3, the matrix assigned hash codes are assigned to themembers of trunk group T1 as follows: hash code 0 to external port 0,hash code 1 to external port 2, hash code 2 to external port 4 hash code3 to external port 12, hash code 4 to external port 16 and hash code 5to external port 17.

A frame received on an external port which is not a member of trunkgroup T1 is forwarded through the multistage switch 100 based on thedestination address included in the received data frame. For example, aframe received on external port 1 for external port 15, is forwardedthrough the matrix of switch devices as follows: logical fan-in/fan-outdevice 200-1 forwards to ingress device 202-1; ingress device 202-1forwards to egress device 204-3 and egress device 204-3 forwards tological fan-in/fan-out device 200-6. The frame is forwarded by eachswitch device dependent on a forward vector computed locally by eachswitch device.

A frame received at external port 1 for trunk group T1 can be forwardedthrough any member of the trunk group. For example, the frame can beforwarded through external port 0 or external port 2 in logicalfan-in/fan-out device 200-1.

The forward vector for a received frame is determined in each switchdevice by combining a plurality of vectors. The forward vector selectsthe ports in the switch device through which the frame is forwarded. Aphysical forward vector selects all the ports through which a receivedframe can be forwarded. An echo suppression vector reduces the number ofavailable ports by ensuring that a frame is not forwarded through theport at which it was received. A trunk member vector selects one of theavailable ports so that frames for the same data flow (same destinationand source address) are forwarded through the same trunk member.

In the example shown in FIG. 3, the frame is forwarded to trunk T1through trunk member 5 on connection 302-3; that is, external port 17 oflogical fan-in/fan-out device 200-6. However, extra frames are alsoneedlessly sent to egress device 204-1, logical fan-in/fan-out device200-2, and logical fan-in/fan-out device 200-5 because a localforwarding decision is made for the frame in each switch device in themultistage switch 300 and every received frame is forwarded to the nextstage. The forwarding decision for forwarding a received frame in eachswitch device is described in more detail in conjunction with FIGS. 4Aand 4B.

FIG. 4A is a block diagram of row 104-1 in the multi-stage switch shownin FIG. 3 illustrating forwarding tables stored in each of the switchdevices. The forwarding decision for a received frame in each switchdevice is dependent on the vectors from the entries selected in threeforwarding tables as explained above; an echo suppression table (EST), aphysical forward vector table and a trunk member vector table. Lookingat fan-in/fan-out device 200-1, external Ports 0, 1, 2 arebidirectional, and internal ports U and D are unidirectional. Echosuppression table 400 stored in logical fan-in/fan-out device 200-1includes an entry for each respective input port; that is, 0, 1, 2, anddownlink port D. Each echo suppression table entry includes a bit foreach output port; that is 0, 1, 2, and uplink port U. A bit is set ‘1’in the echo suppression table entry to indicate that the frame can beforwarded through the respective output port. For example, referring tothe entry for input port 1, all bits are ‘1’ except for port 1, toindicate that a frame received on port 1 cannot be forwarded on port 1,that is, the input port at which the frame was received. Referring tothe entry for port 0, all bits are ‘1’ except ports 0 and 2 because bothports 0 and 2 are members of trunk group T1. Thus, each entry in theecho suppression table performs echo suppression by ensuring that aframe received by a member of trunk group T1 cannot be forwarded throughany other member of trunk group T1.

The physical forward vector (PFV) table 402 in logical fan-in/fan-outdevice 200-1 includes a PFV for the destination address associated withtrunk group T1. The PFV has a bit for each output port; that is, 0, 1,2, and U in the logical fan-in/fan-out device. A bit set to ‘1’ in thePFV indicates that the frame can be forwarded through the respectiveport. Bits for port 0, 2 and U are ‘1’ because there are members of thetrunk group on ports 0, 2 and on other switch devices which can bereached through the U port.

The trunk table 404 in logical fan-in/fan-out device 200-1 includes anentry for each hash value. In trunk 1 there are six members and thus sixhash values. One trunk table is used to forward all frames received bythe logical fan-in/fan-out device. Each entry in the trunk table 404includes a bit for each output port in the fan-in/fan-out device. A bitis set to ‘1’ to indicate that the frame can be forwarded through therespective output port. The entry for hash value=0 indicates that theframe can be forwarded through port 0, 1 and U. The entry for hashvalue=5 indicates that the frame can be forwarded through port 1 and U.

The echo suppression table includes an entry for each input port. Forexample, echo suppression table 400 in logical fan-in/fan-out device200-1 has an entry for input ports 0, 1, 2, and D. Each echo suppressiontable entry has a bit for each output port in the switch device. Forexample, each echo suppression table entry in echo suppression table 400in logical fan-in/fan-out device 200-1 has a bit for output ports 0, 1,2, and U. A bit is set to ‘1’ if a received frame can be forwardedthrough the port. A bit is set to ‘0’ to echo suppress the frame.

Port 0 and Port 2 are members of trunk group 0. Thus, a frame receivedon port 0 cannot be forwarded through port 0 or port 2. Similarly, aframe received at port 2 cannot be forwarded through port 2 or port 1.Referring to echo suppression entries in echo suppression table 400,bits for port 0 and port 2 are set to ‘1’ in the entries for port 0 andport 2 to echo suppress frames received on members of trunk group 1.However, frames received by port 1 can be forwarded to any port otherthan port 1 because only the bit for port 1 is set to ‘0’.

All frames received through ports 0, 1 and 2 which are not echosuppressed are forwarded through the U port. Thus, even if the selectedhash value for the frame is ‘0’ and the frame for trunk group 1 isforwarded through port ‘0’ in logical fan-in/fan-out device 200-1, whichis a member of trunk group 1, the frame is still forwarded needlesslythrough the U port because the U-bit in each entry in the hash table isset to ‘1’.

The forward vector for a frame received on port 1 in logicalfan-in/fan-out device 200-1 with a hash value of 5 is computed bycombining the entry for input port 1 set to ‘1011’ selected in Local EST400, the selected PFV set to ‘1011’ in the PFV table 402 and the entryfor hash value 5 set to ‘0101’ in trunk table 404. A logical ‘AND’ of‘1011’ from local EST 400, ‘1011’ from the PFV table 402 and ‘0101’ fromthe hash table 404 results in a forward vector set to ‘0001’ and theframe is forwarded through the U port on connection 300-1 to ingressdevice 202-1.

Ingress device 202-1 has input ports labeled U0, U1 and output portslabeled 0, 1, 2. Matrix echo suppression table (“EST”) 406 has an entryfor each external port (labeled 0-17) in the multistage switch 300. Eachecho suppression table entry has a bit for each of the ingress device'soutput ports. The entry for port 1 is selected because the frame wasreceived on port 1. PFV Table 408 has a bit for each output port, a bitset to ‘1’ indicates that the frame can be forwarded through therespective output port. The selected PFV in the PFV table 408 indicatesthat a received frame for trunk group T1 can be forwarded through outputport 0 or port 2 because members of trunk group T1 can be reachedthrough output port 0 or port 2. Trunk table 410 has an entry for eachhash value. Each entry in trunk table 410 includes a bit for each outputport 0, 1, 2. The bit is set to indicate that the frame can be forwardedthrough the respective output port 0, 1, 2. The received frame for trunkgroup T1 is forwarded through ports 0 and 2 based on a forward vector of‘101’ computed by combining selected matrix EST vector ‘111’ with PFV‘101’ and trunk member vector ‘111’ using a logical ‘AND’ function.

Egress device 204-1 has three input ports labeled 0, 1, 2 and two outputports labeled D0 and D1. Trunk table 416 has an entry for each hashvalue and each entry has a respective bit for each internal output portin the egress device 204-1. A bit is set to ‘1’ if the frame can beforwarded on the respective port. PFV Table 414 has a bit for eachoutput port, the state of the bit indicates whether the frame can beforwarded through the respective output port. Matrix echo suppressiontable 412 has an entry per external port (0-17) with each entry having abit for each output port, the state of each bit indicating whether theframe can be forwarded through the respective port.

ANDing the echo suppression entry for external port 1 set to ‘01’ withPFV 414 set to ‘11’ and the entry for hash value=5 in trunk table 416set to ‘11’ results in a forward vector of ‘01’ and the frame isforwarded on D1 to logical fan-in/fan-out device 200-2.

Matrix EST 418 in logical fan-in/fan-out device 200-2 has an entry foreach external port (0-17) in the multistage switch 300. Each matrix ESTentry has a bit for each output port 0, 1, 2, U. The bits are set sothat a frame is not forwarded through the port at which it was received.The PFV entry in PFV table 420 in logical fan-in/fan-out device 200-2has a bit for each output port, the state of each bit indicating whetherthe frame can be forwarded to the destination through the respectiveoutput port. Trunk table 422 in logical fan-in/fan-out device 200-2 hasan entry for each hash value, each entry having a bit per output port inthe fan-in/fan-out device 200-2. ANDing the selected entries in eachtable for a frame received on external port 1 results in forward vector‘0000’ and the forward frame is dropped; that is, the frame was notforwarded to any output ports. Thus, the frame was needlessly forwardedthrough egress device 204-1 to logical fan-in/fan-out device 200-2. Theframe is also forwarded through internal outport port 2 in ingressdevice 202-1 to row 104-3.

FIG. 4B is a block diagram of row 104-3 in the multi-stage switch 300shown in FIG. 3. Egress device 204-3 receives the forwarded frame oninput port 0 and determines where to forward the frame dependent onselected entries in tables 424, 426, 428. The entry for external port 1,the external port at which the frame was received is selected in MatrixEST 424. Each Matrix EST entry has a bit for each output port in ingressdevice 204-3. Output port D0 is coupled to logical fan-in/fan-out device200-5 and output port D1 is coupled to logical fan-in/fan-out device200-6. Logical fan-in/fan-out devices 200-5, 200-6 both include membersof trunk group T1. Thus, bits for D0 and D1 in the Matrix EST are set to‘1’ to indicate the received frame can be forwarded. Also, bits from D0and D1 are set to ‘1’ in the selected PFV in PFV table 426 and in theselected trunk table entry in trunk table 428. Thus, the forward vectoris ‘11’ and the frame is forwarded to logical fan-in/fan-out device200-5 on path 302-6 and to logical fan-in/fan-out device 200-6 on path302-3.

In logical fan-in/fan-out device 200-5, the forwarded frame is droppedbecause the trunk table entry for hash value 5 in trunk table 440disables forwarding through external port 12 (port 0 in logicalfan-in/fan-out device 200-5) even though the matrix EST entry forexternal port 1 in matrix EST 436 and the PFV in PFV table 438 for thedestination address enables forwarding through logical fan-in/fan-outdevice port 0.

In logical fan-in/fan-out device 200-6, the frame is forwarded throughfan-in/fan-out device port 2 (labeled external port 17) because thematrix EST entry for external port 1 in the matrix EST table 430, thetrunk table entry for hash value 5 in trunk table 434 and PFV in the PFVtable 432 all allow forwarding through fan-in/fan-out device port 2. Asshown in FIG. 4B, the frame was needlessly forwarded through logicalfan-in/fan-out device 200-5.

FIG. 5 is a block diagram of the multistage switch shown in FIG. 3illustrating trunk aware forwarding according to the principles of thepresent invention. A frame received on external port 1 is forwarded onthe dashed path to ingress device 202-1, egress device 204-3 and logicalfan-in/fan-out device 200-6, but is not forwarded on the dotted path toegress device 204-1 logical fan-in/fan-out device 200-2 and logicalfan-in/fan-out device 200-5. Each switch device having information for amember of the trunk group T1 performs trunk aware forwarding to reducethe number of ports through which a frame is forwarded, resulting inreducing internal data traffic in the multistage switch 300.

FIG. 6A is a block diagram including switch devices in row 104-1 in FIG.5 showing tables providing trunk aware forwarding included in the switchdevices. As shown, switch devices 200-1, 202-1 include a trunk table fortrunk group T1. Each entry in PFV table 602 in logical fan-in/fan-outdevice 200-1 has a table select field. The table select field selects atrunk table dependent on the respective destination address in areceived frame. If the destination address is a trunk, the trunk tablefor the trunk is selected by the trunk table select field. If thedestination address is not a trunk, the default trunk table is selected.The default trunk table is the trunk table that was described inconjunction with FIGS. 4A-4B. Logical fan-in/fan-out device 200-1includes a trunk table 606 for trunk group T1 because external ports 0and 2 are members of trunk T1.

A frame received on external port 1 for trunk T1 selects a PFV entry inthe PFV table 602 for the trunk's destination address. The Table Select(TS) field 608 in the PFV entry set to ‘1’ to select trunk table 606.The trunk table entries for hash value 0 and hash value 1 in trunk table606 differ from the entries in default trunk table 610 so that framesforwarded to the trunk on external port 0 and external port 1 are notunnecessarily forwarded on uplink port U through connection 300-1 toingress device 202-1.

As shown, in trunk table 606 the U bit is set to ‘0’ in the trunk tableentries for hash value=0 and hash value=1, to disable the forwarding offrames on the U port to ingress device 202-1. By providing selectabletrunk tables and a table select field 608, in the PFV entry, frames formembers having a hash value=0 or a hash value=1 of trunk group T1received by logical fan-in/fan-out device 200-1 can be pruned in logicalfan-in/fan-out device 200-1.

Frames that are not destined to trunk T1 members (hash=0, hash=1)logical fan-in/fan-out device 200-1 are forwarded through the U port onpath 300-1 to ingress device 202-1. In ingress switch device 202-1 tableselect 616 in the selected PFV entry selects trunk table 614 for trunkgroup T1. The trunk table entry for hash value=5 forwards of the frameonly through output port 2. Thus the frame is pruned in ingress switchdevice 202-1 by not forwarding the frame through port 0. Ingress switchdevice 202-1 receives frames on internal input port U0 from logicalfan-in/fan-out device 200-1 and on internal input UI from logicalfan-in/fan-out device 200-2. Frames are only forwarded on output port 0when the hash value is 2 and on output port 2 when the hash value is 3,4 and 5. Thus, frames with hash values 0, and 1 received at logicalfan-in/fan-out device 200-1 are pruned at logical fan-in/fan-out device200-1. Also frames received on external ports 3 and 5 for a member oftrunk group T1 by logical fan-in/fan-out device 200-2 with hash values 0and 2 are pruned at ingress device 202-1 so that the frames are onlyforwarded to external ports 0 or 2 through logical fan-in/fan-out device200-1. Frames received by ingress device 202-1 with hash value 2 isforwarded through output port 0 to egress device 204-1. Frames receivedwith hash values 3, 4, and 5 are forwarded through output port 2 to row104-3.

FIG. 6B is a block diagram of row 104-3 shown in FIG. 5 showing tablesproviding trunk aware forwarding included in the switch devices. Egressdevice 204-3 includes trunk tables 618 and 620. Trunk table 618 isselected by Table Select 624 for all frames received for trunk group T1.Frames for hash values 0, 1 or 2 for trunk group T1 are not receivedbecause they are pruned by switch devices in row 104-1 as described inconjunction with FIG. 6A. Thus, neither of the outputs D0 and D1 isselected in trunk table 618 for hash 0, 1, and 2. Frames received forhash value 3 are forwarded to output port D1 because the bit for D1 isset ‘1’. Frames received for hash values 4 and 5 are forwarded tological fan-in/fan-out device 200-6. Thus, frames for trunk group T1 areonly forwarded to logical fan-in/fan-out device 200-5 if the hash valueis 3. Frames with hash values 4 and 5 are pruned by not being forwardedto logical fan-in/fan-out device 200-5 on path 302-6.

Frames for trunk group T1 with hash values 4 and 5 are received byfan-in/fan-out device 200-6 on input port D. Trunk table 626 for trunkgroup T1 is selected by table select 628 when fan-in/fan-out device200-6 receives a frame for trunk group T1. Frames for hash values 4 and5 are not forwarded to the uplink port to ingress device 202-3. Framesfor hash value 5 are forwarded through fan-in/fan-out device port 2(labeled external port 17) and frames for hash value 4 are forwardedthrough fan-in/fan-out device port 1 (labeled external port 16). Thus,trunk aware forwarding allows traffic for a trunk group to be pruned ineach multistage switch device by providing a trunk table for the trunkin each switch device having a member of the trunk and storing a tableselect with the PFV.

FIG. 7 illustrates trunk aware forwarding which prunes a frame in thefan-in/fan-out device at which the frame was received. Trunk group T1has two members on fan-in/fan-out device 200-1; that is, port 0 and port2. The frame is received at port 1 of fan-in/fan-out device 200-1 and isdestined to trunk group T1. The echo suppression vector selected in thelocal echo suppression table 700 for port 1 ensures that the frame isnot forwarded through external port 1. The physical forward vectorselected by the destination address for T1 in the PFV table 702 selectstrunk table 708 for trunk group T1. The computed hash for the sourceaddress and destination address included in the frame is 0. The trunkmember vector in trunk table 708 for “computed hash”=0 selects externalports 0 and 1 for forwarding the frame. The forward vector is computedby combining the selected echo suppression vector, physical forwardvector and the trunk member vector. The forward vector selects externalport 0 for forwarding the frame. Thus, the frame is not forwardedthrough the matrix of switch devices. As a result, no unnecessarytraffic is forwarded through the multistage switch.

FIG. 8A illustrates a prior art frame 800 which may be received at anyof the external ports 132-0, . . . 132-12n in the multistage switch 100(FIG. 1). FIG. 8B illustrates a prior art Ethernet header which may beincluded in the frame 800 shown in FIG. 8A. FIG. 8C illustrates a priorart Internet Protocol (“IP”) header which may be included in the frame800 shown in FIG. 8A.

Turning to FIG. 8A, the frame 800 includes a data payload 810 andheaders for networking layers 802, 804, 806, 808. Headers for four ofthe layers in the Open Systems Interconnection (“OSI”) model are shown,the physical layer (L1) header 802, the data link layer (L2) header 804,the networking layer (L3) header 806 and the transport layer (L4) header808. For example, the data link layer (L2) header 804 may be Ethernetand the networking layer (L3) header 806 may be IP. The frame 800 alsoincludes a checksum 812.

Turning to FIG. 8B, the Ethernet data link (L2) header 804 includes adevice address for the destination 112 (FIG. 1); that is, the L2destination address 814, and a device address for the source 102 a, 102b (FIG. 1); that is, the L2 source address 816, an optional VirtualLocal Area Network Identification (“VLAN ID”) field 818 and alength/type field 820. The VLAN ID 818 includes a Tag ProtocolIdentifier (“TPI”) field 818 a and a Tag Control Information (“TCI”)field 818 b. The VLAN ID field 818 provides support for VLAN switchingbased on IEEE 802.1Q tagging and IEEE 802.ID 1988 (802.1p) prioritylevels.

Turning to FIG. 8C the IP network layer (L3) header 806 includes anetwork address for the source 102 a, 102 b (FIG. 1), that is the IPsource address 844, and a network address for the destination 112 (FIG.1), that is, the IP destination address 846. Other fields in the IPnetwork layer header 806 include Version 822, HLEN 834, Type of Service(“TOS”) 836, Total Length 828, Identification 830, Flags 832, FragmentOffset 834, Time to Live (“TTL”) 836, Protocol field 840, HeaderChecksum 842, Options 848 and pad 850. A frame 800 (FIG. 8A) receivedfrom a source 102 a, 102 b (FIG. 1) is bridged to one or more externalports dependent on the destination address 814 (FIG. 8B) stored in theEthernet data link (L2) header 804 (FIG. 8A) or is routed to one or moreexternal ports dependent on the IP destination address 846 stored in theIP network layer (L3) header 806.

As discussed in conjunction with FIG. 6A, a forwarding decision is madein the switch devices in the multistage switch 100 dependent on a flowhash. Typically, the flow hash is computed based on Media Access ControlSource Address (“MACSA”), Media Access Control Destination Address(“MACDA”), Internet Protocol Destination Address (“IPDA”), InternetProtocol Source Address (“IPSA”), and L2 ports stored in the header ofthe received frame. The MACSA, MACDA, IPDA, IPSA, and L2 have alreadybeen described in conjunction with FIGS. 8A-8C. The flow hash providestemporal ordering for a given flow.

FIG. 9 illustrates vectors and tables in one of the switch devices 200,202, 204 shown in FIG. 2 for computing a forward vector 540 for areceived frame. The forward vector 540 is computed by combining a trunkmember vector 512, a physical forward vector (PFV) 514 and an echosuppression vector 524 for the frame. The PFV 514 selects all the portsthrough which the frame can be forwarded to a destination. The echosuppression vector 524 ensures that an incoming frame is not forwardedto the same port at which it was received or to other ports which aremembers of the same trunk group as the source port. The trunk membervector 512 selects one of the selected ports in the PFV through which toforward the frame to the destination so that frames for the same flow(same source and destination address) are forwarded through the sameexternal port.

The PFV 514 is selected from the forward PFV table 538 dependent on thedestination address 536 included in the received frame. The destinationaddress 536 is either the IP destination address 846 (FIG. 8C) or theMAC DA 814 (FIG. 8B) dependent on the frame type. The PFV 514 includes abit for each output port in the switch device. A port bit is set to ‘1’to enable forwarding to the respective port and set to ‘0’ to disableforwarding to the respective port, thereby indicating all the ports towhich a received frame can be forwarded to the destination. If thedestination can be accessed through a trunk group, a port bit is set to‘1’ in the physical forward vector for all members of the trunk group.

The echo suppression vector 524 ensures that the frame is not forwardedthrough the port through which it was received. The echo suppressiontables 504 includes a local echo suppression table 518 and a Matrix echosuppression table 520. The local echo suppression table 518 ensures thata frame is not forwarded through the local port or a member of the sametrunk as the local port on which it was received. The Matrix echosuppression table 520 ensures that a frame is not forwarded through anyexternal port which is a member of the trunk on which the frame wasreceived.

In a multistage switch which supports trunking, frames can be forwardedto a destination on through any of the members of the trunk group. In amultistage switch which supports trunking across external ports ondifferent switch devices it is necessary to identify which externalports belong to the same trunk group on which the frame was received. Asa result, the external port number; that is the port at which the frameis received is used to index the Matrix echo suppression table 520. Theindexed entry ensures that the frame is not forwarded through a memberof the trunk on which it was received.

The matrix EST table 520 filters frames based on external ports (0-17)to the multistage switch so that a frame is not forwarded to theexternal port of any of the members of a trunk group through which it isreceived. Referring to FIG. 7, the local EST table 700 in logicalfan-in/fan-out device 200-1 filters frames received on local input ports0, 1, 2, D. Referring to FIG. 6B, the matrix EST table 640 in logicalfan-in/fan-out device 200-6 filters frames received on external ports0-17.

Returning to FIG. 9, for example, if external port 0, 2 and 3 are allmembers of the same trunk group, an incoming data frame on external port0 cannot be forwarded to external ports 0, 2 or 3 because they are allpart of the same trunk group. The echo suppression table 504 storesgroup membership information for the trunk and ensures that such echoingwill not occur. The trunk member vector 512 provides a mask for thephysical forward vector (PFV) 514, to select one of the enabled ports,through which to forward the received frame.

The array of trunk tables 530 includes a trunk table 502 for each trunkgroup in the multistage switch having a local member. The flow hashlogic 532 in the switch device creates a computed flow hash 510 derivedfrom the source and destination address included in headers 534 in thereceived frame. The computed flow hash 510 ensures that frames receivedfor the same data flow are forwarded through the same destination. Thearray of trunk tables 530 includes a default trunk table which is usedfor all destination addresses other than the destination address for atrunk group.

The trunk group table for a particular trunk group is used in thefollowing situations: (1) when the frame is destined only to the trunkgroup, (2) the frame is destined to the trunk group and local non-trunkgroups, (3) the frame is destined to the trunk group and another trunkgroup that only has members locally and (4) when the frame is destinedto the trunk group and to another trunk group that has the same hashcode distribution in the trunk table.

The trunk table for a trunk group is selected by a trunk table selector708 included in the PFV. The table selector selects a trunk tabledependent on the respective destination address. If the destinationaddress is a trunk, the trunk table for the trunk is selected by thetrunk table select field. If the destination address is not a trunk, thedefault trunk in the array of trunk tables 530 is selected.

Each trunk member has a “matrix assigned” hash code. A received frame isforwarded towards a trunk member if the “matrix assigned hash code”matches the “computed hash code” computed by the flow hash logic 532.

A frame may be destined to two or more groups that do not use the samehash code distribution in the Trunk Table 502. Therefore, more TrunkTables 502 are required when frames are sent to two or more trunkgroups. The absolute maximum number of Trunk Tables 502 required is2^(n)+1, where ‘n’ is the maximum number of trunk groups subtended bythe switch device and ‘1’ is the default trunk table. For example, in aningress device 202-1 (FIG. 3) with n=6 ingress ports, there is a maximumof six trunk groups subtended by the device and all six trunk groupsrequire different hash distribution on the uplink port. Thus, theabsolute maximum number of Trunk Tables is 2⁶+1=65.

If a trunk group shares a single Trunk Table 502 with traffic destinedto non-trunk ports or other trunk ports, the ingress device 202-1 mustsend a received frame destined to a member of a trunk group to allegress devices that include members of the trunk group. Thus, if a trunkgroup has members on twelve egress devices, the received frame must beforwarded to all twelve egress devices.

The number of egress devices 204-1 (FIG. 2) to which the received framemust be forwarded can be reduced by providing selectable trunk tables530. However, the maximum number of Trunk Tables 532 is much larger thanthat required for an ingress device 202-1 (FIG. 4). For example, if eachegress device 204-1 (FIG. 2) subtends six different trunk groups, themaximum number of tables is 2^((12×6))+1 in a multi-stage switchincluding a fan-in/fan-out device 200-1 with six ports and twelveingress devices 202-1. The number of tables is much larger if thefan-in/fan-out device has 24 ports.

The number of selectable trunk tables 530 can be reduced by using fixedhash code distributions. Trunk groups are spread across the ingressdevices 202. Trunk groups that do not have slots in common in the trunktables 530 can be placed in the same trunk table without causing overlapof hash codes. However, trunk groups that reside on the same ingressdevice 202 may have hash code overlap. If the trunk groups on the sameingress device 202 use the same hash codes, there is no overlap and thetrunk groups may share a trunk table 502 without causing superfluoustraffic in the matrix. Hash code combinations are restricted by usingfixed distributions for all trunk groups that have the same number ofslots in common.

For example, if trunk group 1 and trunk group 2 have ports on ingressdevice 4 in common; that is, hash codes 32-63. The two trunk groups canshare a single trunk table by using fixed hash code distributions.

FIG. 10A illustrates two trunk groups 550 a, 550 b sharing a singletrunk table 552, each of the trunk groups having a different hash codedistribution. Trunk group T1 550 a has members on rows 3 and 4. Row 3uses hash codes 0-31 and row 4 uses hash codes 32-63. Trunk group T2 550b has members on row 4 and row 6. Row 4 uses hash codes 24-63 and row 6uses hash codes 0-23. The total number of trunk tables required perswitch device is reduced by sharing trunk tables.

In order to share a single table, row 4 is selected for all hash codescommon to trunk group T1 and trunk group T2; that is, all codes between24 and 63. However codes 24-31 are also stored for row 3 as shown. Thisresults in unnecessary traffic through the matrix because a framedestined to trunk T1 with hash codes between 24 and 31 is sent to bothrow 3 and row 4. The frame destined to row 4 is not required and issubsequently dropped by either the egress device or the fan-in/fan-outdevice.

FIG. 10B illustrates the two trunk groups shown in FIG. 10A with eachtrunk group having the same hash code distribution. Two trunk groups canshare a single trunk table to reduce the number of trunk tables requiredin a switch device. A trunk table can only be shared if the middlecolumn values in the trunk table are the same. As shown, trunk group T2uses the same hash code distribution as trunk group T1. Thus, codes24-31 are moved from shared row 4 to row 6 for trunk group T2. Aftermoving codes 24-31, only shared routes which also share the same row arestored in row 4; that is, the middle column values are the same. Withthis distribution of hash codes for trunk groups T1 and T2, there is noneedless frame duplication for frames destined to trunk group T1 andtrunk group T1 and T2 can share the same trunk table.

In an embodiment with three rows, hash codes can be distributed asfollows: hash codes 0-20 for the first row, hash codes 21-41 for thesecond row and hash codes 42-63 for the third row. A total of 10 sharedtrunk tables are required for 220 configurations. In an embodiment withfour rows, hash codes can be distributed as follows: hash codes 0-15 forthe first row, hash codes 15-31 for the second row, hash codes 32-57 forthe third row and hash codes 48-63 for the fourth row. A total of 21shared trunk tables are required for 495 configurations.

FIG. 11 illustrates an example of local echo suppression tables forreducing unnecessary traffic through the matrix for received unicastframes. The received unicast frames are destined to the same trunk groupand the trunk group has members that are reached across the matrix.

The fan-in/fan-out device 200-1 can echo suppress frames received anddestined to the same trunk group even when the trunk group has membersthat are reached across the matrix. This only applies to unicast frames.To suppress frames received for the same trunk group, eachfan-in/fan-out device 200-1 has a plurality of echo suppression tables900-1, 900-2, one per trunk group. For example, six echo suppressiontables are required for a fan-in/fan-out device 200-1 with six portsbecause there can be a maximum of six multicast groups.

Thus, the selected PFV 906 in the PPFV table 904 selects a trunk table902-1, 902-2 and an echo suppression table 902-1, 902-2 based on theswitch input port at which the frame is received. If the frame isreceived by a member of a trunk group port and is destined to anotherport in the same trunk group, the frame is echo suppressed by notforwarding the frame through the uplink port of the fan-in/fan-outdevice 200-1.

While this invention has been particularly shown and described withreferences to preferred embodiments thereof, it will be understood bythose skilled in the art that various changes in form and details may bemade therein without departing from the scope of the inventionencompassed by the appended claims.

1. A method for forwarding received data for a logical link, the methodimplemented in a switch device in a multistage switch, the multistageswitch comprising a matrix of switch devices connected to each otherthrough internal output ports, the logical link comprising a pluralityof physical links and coupling the multistage switch to a destinationthrough a plurality of output ports connected to the multistage switch,the method comprising: upon determining at a switch device of the matrixof switch devices to forward the received data to one of the physicallinks of the logical link, selecting a trunk table associated with thelogical link from a plurality of trunk tables, each trunk tableincluding trunk table entries; and computing a forward vector for thereceived data dependent on a selected trunk table entry for the receiveddata, the forward vector indicating where the received data is to beforwarded forwarding the received data to the internal output portindicated by the forward vector, and the forward vector indicating theinternal output port through which to forward the received data.
 2. Themethod as claimed in claim 1 wherein each trunk table entry includes abit for each internal output port of the switch device.
 3. The method asclaimed in claim 1 wherein said forward vector is computed using theforwarding switch device.
 4. The method as claimed in claim 1 furthercomprising storing an echo suppression vector for each internal port ofthe multistage switch.
 5. The method as claimed in claim 1 furthercomprising: receiving a frame that is associated with the destination;identifying whether the destination is coupled by the logical link; andif and only if the destination is coupled by the logical link, using atrunk member vector provided in one of the trunk tables to maskforwarding bits of a forwarding vector.
 6. The method as claimed inclaim 5 further comprising selecting a vector dependent on the internalport receiving the frame so that the frame is not forwarded to anyphysical links of a logical link on which it was received.
 7. The methodas claimed in claim 5 wherein the frame includes an IP network layerheader.
 8. The method as claimed in claim 7 wherein the forwardingvector is a function of information in the IP network layer header ofthe frame.
 9. The method as claimed in claim 5 further comprisingindexing a flow hash for the frame dependent on a destination addressand a source address included in the frame.
 10. A multistage switch towhich a logical link couples a destination, the logical link comprisinga plurality of physical links, the multistage switch comprising: aplurality of external ports, each physical link coupled to one of theplurality of external ports; and a matrix of coupled switch devices, aframe received for the destination being forwarded through at least oneof the switch devices to one of the physical links in the logical link,the switch device which receives the forwarded frame forwarding theframe based on the logical link toward less than all of the physicallinks of the logical link to reduce the number of subsequent switchdevices through which the frame is forwarded, a row of a number of thecoupled switch devices including at least one output-port coupleddevice, an ingress device having internal input ports connected to saidoutput-port coupled device and internal output ports connected to anegress device, the egress device having internal input ports connectedto said ingress device and internal output ports connected to at least aselected one of said output-port coupled device and another output-portcoupled device.
 11. The switch as claimed in claim 10 wherein saidoutput-port coupled device is a fan-in/fan-out device coupled to outputports of the multistage switch.
 12. The multistage switch as claimed inclaim 10 wherein the ingress device stores an echo suppression table.13. The multistage switch as claimed in claim 12 wherein the echosuppression table includes an entry for each port, the entry selecteddependent on the port receiving the frame and the entry ensuring thatthe frame is not forwarded to any physical links of the logical link onwhich it was received.
 14. The multistage switch as claimed in claim 12wherein said echo suppression table includes an entry for each externalport at which said frame is received.
 15. The multistage switch asclaimed in claim 10 wherein the ingress device includes flow hash logicthat indexes a flow hash for the frame dependent on a destinationaddress and a source address included in the frame.
 16. The multistageswitch as claimed in claim 10 wherein the ingress device includes atrunk table selector that selects a trunk table for the logical link toreduce the number of ports of the multistage switch through which toforward the frame.
 17. The multistage switch as claimed in claim 16wherein the trunk table for the logical link is shared by anotherlogical link.
 18. A multistage switch to which a logical link couples adestination, the logical link comprising a plurality of physical links,the multistage switch comprising: a plurality of external ports, eachphysical link coupled to one of the plurality of external ports; amatrix of coupled switch devices, at least two of the physical linkscoupled to external ports on different switch devices, a row of a numberof the coupled switch devices including at least one switch meanscoupled to output ports of the multistage switch, an ingress meanshaving internal input ports connected to said switch means and internaloutput ports connected to an egress means, the egress means havinginternal input ports connected to said ingress means and internal outputports connected to at least a selected one of said switch means andanother switch means; and means for forwarding a frame received for thedestination through at least one of the coupled switch devices to one ofthe physical links in the logical link based on the logical link towardless than all of the physical links of the logical link to reduce thenumber of subsequent switch devices through which the frame isforwarded.
 19. The multistage switch as claimed in claim 18 furthercomprising means for storing an echo suppression vector for eachinternal port of the multistage switch.